Why is the gap 13 days and not a round number?

The gap started at 10 days in 1582 and has grown by one day each century when the Gregorian calendar skipped a leap year (1700, 1800, 1900) but the Julian calendar did not. That makes 13 days for the 20th and 21st centuries.

Will the gap ever change again?

Yes, the gap will grow to 14 days in 2100. This is because 2100 is a leap year in the Julian calendar but not in the Gregorian calendar.

Do all Orthodox churches use the Julian calendar?

No, some Orthodox churches use the Revised Julian calendar or the Gregorian calendar. But several, including the Russian and Serbian Orthodox churches, still use the Julian calendar for religious dates.

How do I know if a historical date is Old Style or New Style?

The date will often be marked as O.S. (Old Style) or N.S. (New Style). If it is not marked, you need to know the country and the date of its calendar change. For example, Russia switched to the Gregorian calendar in 1918, so dates before that are Julian.

Julian vs Gregorian Calendar: Why the Dates Differ by 13 Days

The Julian calendar runs 13 days behind the Gregorian calendar. Blame the leap-year rules. The Julian calendar adds a leap year every four years, making its average year 365.25 days. The Gregorian calendar skips leap years in century years not divisible by 400, giving an average year of 365.2425 days. Over centuries, that small difference added up. Several Orthodox churches still use the Julian calendar for religious dates, so their Christmas (25 December Julian) falls on 7 January Gregorian. When you see dates marked as Old Style (O.S.) or New Style (N.S.), you know which calendar was used.

How the 13-Day Gap Built Up

The core difference is the length of the average year. The Julian calendar uses 365.25 days, which is too long. The Gregorian calendar uses 365.2425 days, much closer to the actual solar year. That extra length in the Julian calendar caused it to drift ahead of the seasons. By the time the Gregorian calendar was introduced, the drift had reached 10 days. The Gregorian reform removed those 10 days to realign the calendar with the equinox. Since then, the Julian calendar has continued to drift. Because 2100 is a leap year in the Julian calendar but not in the Gregorian, the gap will grow to 14 days in 2100.

Leap Year Rules Compared

The two calendars decide leap years differently.

Julian calendar: A leap year every 4 years. Any year divisible by 4 is a leap year.
Gregorian calendar: A leap year every 4 years, except century years not divisible by 400. So a certain century year was not a leap year in the Gregorian calendar, but it was in the Julian. 2000 was a leap year in both. 2100 will be a leap year in the Julian calendar but not in the Gregorian.

This difference is why the gap increases over time. The Julian calendar gets an extra day every century or so that the Gregorian calendar does not.

Where the Julian Calendar is Still Used

Orthodox Christmas and Church Calendars

Several Orthodox churches still use the Julian calendar to set religious holidays. That is why their Christmas, 25 December in the Julian calendar, falls on 7 January in the Gregorian calendar. The 13-day gap is the sole reason for that shift. Other Orthodox churches have adopted the Revised Julian calendar or the Gregorian calendar, but the Julian calendar remains in use for some church calendars. It also survives among traditionalist groups and in a few remote areas.

Old Style and New Style Dates

When you read historical documents, dates may be marked Old Style (O.S.) or New Style (N.S.). Old Style means the Julian calendar. New Style means the Gregorian. Russia switched to the Gregorian calendar in 1918. A date before 1918 in Russia might appear in Old Style; add 13 days to get the New Style date.

How to Convert a Date from Old Style to New Style

Conversion Steps for the 20th and 21st Centuries

To convert a Julian calendar date (Old Style) to the Gregorian calendar (New Style), add the current gap. For dates in the 20th and 21st centuries, that gap is 13 days. Here is the procedure:

Identify the date in the Julian calendar. For example, 25 December Julian.
Confirm the gap. For any date in those centuries, use 13 days.
Add the gap to the Julian date. 25 December plus 13 days lands on 7 January.
The result is the Gregorian date. 25 December Julian equals 7 January Gregorian.

Gaps for Other Centuries

After the 21st century, the gap becomes 14 days. Before the 20th century, the gap was smaller. In 1700 the gap was 11 days. Always check which century your date falls in before you convert.

Why 2100 Changes the Gap

The gap is not permanent. The year 2100 is a leap year in the Julian calendar but not in the Gregorian. That means the Julian calendar gets an extra day that the Gregorian skips. After 28 February 2100 in the Gregorian calendar, the gap becomes 14 days. This shifts every date calculated after that point. After 2100, Christmas in the Julian calendar (25 December) will fall on 8 January Gregorian.

Historical Calendar Switches

When countries switched from the Julian to the Gregorian calendar, they skipped days. When Russia switched in 1918, the calendar jumped from 31 January 1918 straight to 14 February 1918, dropping 13 days. That is why dates around a switch confuse people. You must know which calendar was in force at the time.

Practical Example Orthodox Christmas

Orthodox churches on the Julian calendar celebrate Christmas on 25 December Julian. The 13-day gap puts that date on 7 January in the Gregorian calendar. This is why many countries with Orthodox traditions mark Christmas on 7 January. The date itself is unchanged in the Julian calendar; it only appears different through a Gregorian lens. The same logic applies to other religious holidays, such as Easter, which some churches calculate using the Julian calendar.

Check the Current Date in Both Calendars

To see today's date in both the Julian and Gregorian calendars, open a site like datetoday.now. It shows the current date, the week number, and the day of the year. Compare the Julian and Gregorian dates side by side. If today is 7 January Gregorian, it is 25 December Julian. This is the fastest way to check the 13-day gap and see how the two calendars align on any given day.